Aluminum hydroxide aggregated particles, process for producing the same, vessel used therefor, and process for producing aluminum hydroxide powder

ABSTRACT

Aluminum hydroxide aggregated particles which have an average particle diameter of not less than 40 μm, an average particle diameter as determined after pressing at 1,000 kg/cm 2  of not more than 35 μm, and an L value of slurry obtained by mixing 20 ml of glycerol and 10 g of the aluminum hydroxide aggregated particles of not more than 69, are obtained by a process comprising the steps of: (a) feeding a supersaturated aqueous sodium aluminate solution to a vessel, (b) adding aluminum hydroxide seed to the supersaturated aqueous sodium aluminate solution, (c) stirring the seed-added solution n the vessel while continuously feeding an additional supersaturated aqueous sodium aluminate solution into the vessel to hydrolyze the supersaturated aqueous sodium aluminate solution, (d) separating the aluminum hydroxide aggregated particles from the aqueous sodium aluminate solution, and (e) continuously discharging the aqueous sodium aluminate solution out of the vessel.

This application is a Divisional of co-pending application Ser. No.10/289,429, filed on Nov. 7, 2002, the entire contents of which arehereby incorporated by reference and for which priority is claimed under35 U.S.C. § 120.

BACKGROUND OF THE INVENTION

This invention relates to a process for producing aluminum hydroxidepowder which makes it possible to obtain a filler-containing resincomposition having an improved transparency, aluminum hydroxideaggregated particles which are the material therefor, a process forproducing the same, and a vessel used therefor.

Aluminum hydroxide powder is often used as a filler for resins, such asunsaturated polyester resin, in producing artificial marble and thelike. When aluminum hydroxide powder is used as a filler for artificialmarble and the like, the powder is required to be excellent in fillingproperty for resins and, at the same time, not to impair thetransparency of resulting resin composition.

As to an industrial process for producing aluminum hydroxide, there arehitherto been known a process which comprises hydrolyzing asupersaturated aqueous sodium aluminate solution in the presence ofseeds. For example, JP 63-23131 B discloses a process which comprisesconnecting plural vessels in series, feeding a supersaturated aqueoussodium aluminate solution continuously to the first vessel, hydrolyzingthe supersaturated aqueous sodium aluminate solution in the presence ofseeds, and allowing hydrolysis to proceed while sending the solutionsuccessively to the second vessel and the third vessel, to obtainaluminum hydroxide.

However, even when the aluminum hydroxide obtained by theabove-mentioned process is filled in resins, it has been impossible toobtain a resin composition having a sufficient transparency.

The object of this invention is to provide aluminum hydroxide aggregatedparticles which can be suitably used for producing aluminum hydroxidepowder which can give a resin composition that shows a high transparencywhen filled in resins or the like, a process for producing theaggregated particles, a vessel used therefor, and a process forproducing aluminum hydroxide powder which uses the aggregated particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing one embodiment of a vesselaccording to this invention.

FIG. 2 is a schematic transverse sectional view of the vessel shown inFIG. 1.

The reference numerals in the Figures signify the following.

1 concentrating region, 2 clarifying region, 3 vessel, 4 baffle plate, 5stirring blades, 6 rotating shaft, 7 stirrer, 8 sweeper, 9 offtake port,10 anticorrosive material, 11 straightening vane

SUMMARY OF THE INVENTION

The present inventors have made extensive study to solve theabove-mentioned problems, and resultantly completed this invention.

Thus, according to this invention, there are provided aluminum hydroxideaggregated particles, which have an average particle diameter of notless than 40 μm, an average particle diameter as determined afterpressing at 1,000 kg/cm² of not more than 35 μm, and an L value ofslurry obtained by mixing 20 ml of glycerol and 10 g of the aluminumhydroxide aggregated particles of not ore than 69.

According to this invention, there is further provided a process forproducing aluminum hydroxide aggregated particles comprising the stepsof:

-   (a) feeding a supersaturated aqueous sodium aluminate solution to a    vessel,-   (b) adding aluminum hydroxide seeds to the supersaturated aqueous    sodium aluminate solution to form a seed-added solution in the    vessel,-   (c) stirring the seed-added solution in the vessel while    continuously feeding an additional supersaturated aqueous sodium    aluminate solution into the vessel to hydrolyze the supersaturated    aqueous sodium aluminate solution to obtain aluminum hydroxide    aggregated particles and an aqueous sodium aluminate solution,-   (d) separating the aluminum hydroxide aggregated particles from the    aqueous sodium aluminate solution, and-   (e) continuously discharging the aqueous sodium aluminate solution    out of the vessel.

According to this invention, there is further provided a process forproducing aluminum hydroxide powder which comprises disintegrating thealuminum hydroxide aggregated particles obtained as above.

According to this invention, there is further provided a vessel used forthe above-mentioned process for producing aluminum hydroxide aggregatedparticles, this is, a vessel having a hollow space defined by asurrounding wall and a bottom part, wherein the vessel has:

-   -   a supply port at a lower portion of the wall,    -   two or more baffle plates mounted on an inner surface of the        wall so that the baffle plates protrude toward the hollow space        and extend in vertical direction along the inner surface of the        wall from immediately above the bottom part to a prescribed        height from the bottom part, and    -   a stirring blade located within the hollow space and within a        space below the prescribed height of the baffle plates.

DETAILED DESCRIPTION OF THE INVENTION

The aluminum hydroxide aggregated particles (hereinafter referred to as“aggregated particles”) of this invention have an average particlediameter of 40 μm, or more, preferably 50 μm or more, more preferably 60μm or more, and has an average particle diameter, as determined afterpressed at 1,000 kg/cm², of not more than 35 μm. The aggregatedparticles are each an assembly of at least 2, preferably 8 or moreprimary particles. For the aggregated particles, the average particlediameter determined after pressing is smaller than the average particlediameter before pressing, and the difference of average particlediameter before and after pressing is usually not less than 5 μm. Theface that the average particle diameter determined after pressing issmaller indicates that the cohesive force of the aggregated particle isweak and the particle is easily disintegrated to yield primaryparticles. The average particle diameter mentioned above can bedetermined with a laster scattering type particle distribution measuringapparatus.

In the aggregated particles of this invention, a slurry obtained bymixing 20 ml of glycerol and 10 g of aggregated particles shows an Lvalue of not more than 69 in the Lab indication system specified byCommission International de l'Eclairage. Aggregated particles showing anL value higher than 69, even when they are disintegrated and filled inresins, cannot give a resin composition having a high transparency. TheL value of aggregated particles is the smaller the better, and is, forexample, preferably not more than 65, more preferably not more than 63.

The aggregated particles of this invention having characteristicproperties shown above can be obtained, for example, by a process whichcomprises the steps of (a) feeding a supersaturated aqueous sodiumaluminate solution to a vessel, (b) adding aluminum hydroxide seeds(hereinafter abbreviated as “seeds”) to the supersaturated aqueoussodium aluminate solution to form a seed-added solution in the vessel,(c) stirring the seed-added solution in the vessel while continuouslyfeeding an additional supersaturated aqueous sodium aluminate solutioninto the vessel, to hydrolyze the supersaturated aqueous sodiumaluminate solution to obtain aggregated particles, (d) separating theaggregated particles from the aqueous sodium aluminate solution, and (e)continuously discharging the aqueous sodium aluminate solution out ofthe vessel.

In step (a), the supersaturated aqueous sodium aluminate solution fedinto the vessel preferably has an effective Na₂O (caustic Na₂O)concentration of about 120-180 g/l, an Al₂O₃ concentration of about120-180 g/l and a molar ratio (Na₂O/Al₂O₃) of about 1.2-1.8. The term“effective Na₂O” refers to a value obtained by subtracting Na₂CO₃content (in terms of Na₂O) from the total Na₂O content in the aqueoussodium aluminate solution. The supersaturated aqueous sodium aluminatesolution can be prepared, for example, by a method which comprisesmixing bauxite with an aqueous sodium hydroxide solution, heating theresulting mixture at 120° C. or above to extract the alumina componentin the bauxite, then subjecting the mixture to separation for examplewith a thickener, filtering the aqueous sodium aluminate solution thusobtained, and cooling the filtrate, or a method which comprises mixingaluminum hydroxide with sodium hydroxide, heating the mixture at 120° C.or above to dissolve aluminum hydroxide, subjecting the mixture toseparation, e.g., filtration, and cooling the aqueous sodium aluminatesolution thus obtained. The supersaturated aqueous sodium aluminatesolution can also be prepared by a method which comprises mixingaluminum hydroxide with a saturated aqueous sodium aluminate solutionafter hydrolysis or an unsaturated aqueous sodium aluminate solutionheating the mixture at 120° C. or above to dissolve aluminum hydroxide,followed by filtration, and cooling the aqueous sodium aluminatesolution thus obtained.

The seeds added in step (b) preferably have an average particle diameterof about 1-2 μm. The seeds prepared by a method which comprises, withoutresorting to grinding, adding an acid aluminum salt, such as aluminumsulfate, to an aqueous sodium aluminate solution to cause hydrolysis aremore preferable than those obtained by grinding coarse aluminumhydroxide. Addition of seeds can shorten the indication period whichelapses till aluminum hydroxide is formed by hydrolysis of thesupersaturated aqueous sodium aluminate, as well as control the particlediameter of aluminum hydroxide powder ultimately obtained. The amount ofseeds can be appropriately determined according to the particle diameterof the intended aluminum hydroxide powder.

The stirring in step (c) is conducted, for example, by using amechanical stirrer. The peripheral velocity of the stirring blade ispreferably 0.1 m/s or more. The peripheral velocity is preferably notmore than 5 m/s.

The separation in step (d) can be conducted, for example, by a methodwhich utilizes specific gravity difference between the aggregatedparticles and the aqueous sodium aluminate solution. Through theseparation, usually the aggregated particles move toward the lower partof the vessel and the aqueous sodium aluminate solution moves toward theupper part of the vessel, so that the aggregated particles sediment atthe lower part of the vessel to give a slurry having a high solidconcentration. The slurry has a concentration of solid (aggregatedparticles) of preferably 600 g/l or more, more preferably 700 g/l ormore, and preferably not more than 1000 g/l, more preferably not morethan 900 g/l.

In step (e), the aqueous sodium aluminate solution discharged out of thevessel preferably has an effective Na₂O (caustic Na₂O) concentration ofabout 120-180 g/l, an Al₂O₃ concentration of about 60-80 g/l and a molarratio (Na₂O/Al₂O₃) about 2-3.5

The above-mentioned steps (a), (b), (c), (d) and (e) are preferablyconducted in a single vessel. One example of the vessel used herein isshown in FIG. 1. The vessel 3 has at its lower part a supply port (notshown in the Figure) and baffle plates 4. The baffle plates 4, as shownin FIG. 2, protrude from the inner circumferential surface of the wallof vessel 3 and are provided in plurality (e.g., 2-10) at predeterminedintervals in the peripheral direction of vessel 3. Each of the baffleplates 4 extrudes from immediately above the bottom part of vessel 3long along the vertical direction (the direction of the rotating shaft)of vessel 3. By baffle plates 4 thus provided, stirring efficiency isimproved and, at the time of stirring, an ascending current is formedalong the inner surface of the wall of vessel 3, whereby the slurry inthe concentrating region 1 specified by the height of baffle plates 4can be brought to a substantially complete mixings state. As the resultof the aqueous sodium aluminate solution being hydrolyzed in theconcentrating region 1 at the complete mixing state, aggregatedparticles which have only a weak cohesive force and are easilydisintegrated can be obtained. The reachable height of the ascendingcurrent is influenced by the height of baffle plates 4. The more theheight of baffle plates 4 is increased, the higher point the ascendingcurrent reaches, and the concentrating region 1 increases in size.Usually the upper end of baffle plates 4 and the upper end of theconcentrating region 1 are approximately at the same level. The heightof baffle plates 4 is, relative to the total height of the vessel 3,preferably 50% or more, more preferably 70% or more, most preferably 75%or more, and preferably not more than 90%, and more preferably not morethan 80%. Vessel 3 has a stirrer 7 which consists of a stirring blade 5and a rotating shaft 6 which drives blade 5. Stirring blade 5 is, forexample, a stirring vane and is provided in concentrating region 1.Rotating shaft 6 is usually provided at the center of the section,perpendicular to the longitudinal direction, a vessel 3. By the rotationof stirring blade 6, the slurry of concentrating region 1 is stirred. Atthe lower end part of rotating shaft 6, a sweeper 8 is provided. Withthe aid of sweeper 8, sedimentation of aggregated particles to thebottom part of vessel 3 can be prevented.

When the aggregated particles are produced by using vessel 3, in vessel3 are formed a concentrating region 1 and, above concentrating region 1,a clarifying region 2. With vessel 3 along, in addition to thehydrolysis of the supersaturated aqueous sodium aluminate solution andthe separation of the aggregated particles from the aqueous sodiumaluminate solution, concentration of the aggregated particles can alsobe conducted.

The internal circumferential surface of the wall of vessel 3 below theheight not higher than baffle plates 4 and the inner surface of thebottom part of the vessel are preferably lined with an anticorrosivematerial 10. The anticorrosive material used can be, for example,metallic materials, such as stainless steel, nickel, nickel alloy andtitanium; inorganic materials, such as ceramics; and organic materials,such as fluororesins. The lining can be conducted, for example, by amethod of flame-coating or baking the above-mentioned metallic materialsor inorganic materials; by a method of welding or adhering aplate-formed metallic material having a thickness of 1 mm or more,preferably 2 mm or more, and not more than 10 mm, preferably not morethan 3 mm; or by a method of adhering an organic material. Withanticorrosive material 10 being thus lined, the corrosion of theinternal surface of the wall of vessel 3 which is in contact withconcentrating region 1 can be prevented and the coloring of aluminumhydroxide powder ultimately obtained can be decreased. For example, when10 g of methyl methacrylate and 18 g of the aluminum hydroxide powderobtained are mixed, the resulting slurry shows a b value of not morethan 3, preferably not more than 2, in the Lab indication systemspecified by Commission International de l'Eclairage. The smaller bvalue indicates the lower coloring.

On the inner circumferential surface of the wall of vessel 3 contactingwith clarifying region 2 are provided in protrusion of a plurality(e.g., 2-8) of straightening vanes 11. These straightening vanes 11 playthe role of promoting the separation of the aqueous sodium aluminatesolution and the aggregated particles, formed by hydrolysis, from eachother in concentrating region 1 and improving the clarity of the aqueoussodium aluminate solution. The number and the size of the straighteningvane 11 are not particularly limited so long as the clarifying effectfor the liquid is not impaired. Since the aggregated particles aresubstantially not present in clarifying region 2, the inner surface ofvessel 3 contacting with clarifying region 2 needs not be lined withanticorrosive material 10. The aqueous sodium aluminate solution inclarifying region 2 is discharged from the discharge port (not shown inthe Figure) provided at the upper part of vessel 3 to the outside ofvessel 3.

In producing aluminum hydroxide powder by using the above-mentionedvessel, first a predetermined amount of a supersaturated aqueous sodiumaluminate solution is fed into vessel 3 so that the content of thevessel can be stirred. Seeds are added to vessel 3, stirrer 7 is driven,then a supersaturated aqueous sodium aluminate solution is continuouslyfed to vessel 3 and, while the content is being stirred, thesupersaturated aqueous sodium aluminate solution is hydrolyzed. When thefeeding of the supersaturated aqueous sodium aluminate solution tovessel 3 is continued, the liquid level rises and reaches the upper endof baffle plate 4 and, when the feeding is further continued, reachesthe discharge port provided at the upper part of the vessel 3. In vessel3, as the result of separation, aggregated particles and an aqueoussodium aluminate solution are obtained. The solid (aggregated particles)concentration in concentrating region 1 increases gradually. On theother hand, the aqueous sodium aluminate solution is discharged from thedischarge port provided at the upper part of vessel 3 to the outside ofvessel 3. Thereafter, an operation which comprises feeding asupersaturated aqueous sodium aluminate solution from the lower part ofvessel 3 and discharging the same amount of an aqueous sodium aluminatesolution is conducted continuously. By this operation, the solidconcentration in concentrating region 1 increases according to theamount of supersaturated aqueous sodium aluminate solution fedcontinuously. Through the above-mentioned series of operation, thetemperature of vessel 3 is kept at 45° C. or above, preferably at 50° C.or above, and at 80° C. or below, preferably at 60° C. or below. At thetime when the average particle diameter of aggregated particles hasreached a predetermined value (for example 80 μm), the feeding ofsupersaturated aqueous sodium aluminate solution is discontinued, andthe reaction mixture is kept for a predetermined time with stirring. Atthis time, the solid concentration of concentrating region 1 ispreferably 600 g/l or more, more preferably 700 g/l or more, andpreferably not more than 1,000 g/l, more preferably not more than 900g/l. The time during which the supersaturated aqueous sodium aluminatesolution is fed, though it varies depending on the intended particlediameter, is preferably not less than 500 hours and preferably not morethan 1,000 hours. The aggregated particles in concentrating region 1 istaken out of offtake port 9 provided at the bottom of vessel 3,separated from liquid by centrifugation, filtration, or the like, andthen washed according to necessity.

Then the aggregated particles are disintegrated to yield aluminumhydroxide powder. The disintegration is preferably conducted by a methodwhich can break the bond between a primary particle and another primaryparticle without substantially destroying the primary particlethemselves which constitute an aggregated particle, and preferablyconducted, for example, with a kneader, blender, extruder, or the like.The aluminum hydroxide powder can be subjected, according to necessity,to drying or surface treatment. The aluminum hydroxide powder thusobtained has an average particle diameter of preferably not less than 10μm and preferably not more than 35 μm, and can be suitably used as afiller for resins, such as unsaturated polyester resin, acrylic resinand epoxy resin.

EXAMPLE 1

Preparation of Aggregated Particles

A supersaturated aqueous sodium aluminate solution having a temperatureof 58° C., a Na₂O concentration of 125 g/l, an Al₂O₃ concentration of125 g/l and a molar ratio of 1.65 was fed at a flow rate of 100 parts byweight/hour to vessel 3 having a structure shown in FIG. 1. The innerwall of the vessel 3 is provided with baffle plates 4 each having aheight corresponding to 75% of the total height of vessel 3. When theliquid level of the supersaturated aqueous sodium aluminate solutionreached the lower end of the upper stirring blade of blades 5 having twoupper and lower stirring blades, rotation of stirrer 7 was started, and150 parts by weight of seeds having an average particle diameter of 1.1μm were added. While rotating stirrer 7 so that the peripheral velocityof stirring blades 5 might be 0.5 m/s, feeding of the supersaturatedaqueous sodium aluminate solution to concentrating region 1 of vessel 3was continued to allow hydrolysis to proceed in the concentratingregion, and the aqueous sodium aluminate solution was discharged fromthe upper end of vessel 3. The discharged aqueous sodium aluminatesolution had an Na₂O concentration of 125 g/l, an Al₂O₃ concentration of65 g/l and a molar ratio of 3.2. When the average particle diameter ofaggregated particles in concentrating region 1 reached 80 μm, feeding ofthe supersaturated aqueous sodium aluminate solution was discontinued,and the reaction system was kept as it was. The solid concentration inconcentrating region 1 at the time of discontinuing the feed was 800g/l. After being kept, the content was withdrawn from offtake port 9 ofvessel 3, subjected to solid-liquid separation using a centrifugalseparator, and the resulting solid was washed to obtain the aggregatedparticles.

Evaluation of Aggregated Particles

The L value of a slurry obtained by mixing 10 g of the aggregatedparticles obtained above and 20 ml of glycerol was determined with acolor-difference meter (Type A-300, a trade name, mfd. by NipponDenshoku Kogyo K.K.). The result obtained is shown in Table 1.Separately, 5 g of the above-mentioned aggregated particles were placedin a cylindrical die 20 mm in diameter and pressed under a pressure of1,000 kg/cm² for 1 minute, the resulting pellets were pulverized withhands and a roller rod, and then the average particle diameter of theresulting powder was determined. The result obtained is shown in Table2. The term “rate of change” in Table 2 indicates the decrease ofaverage particle diameter observed after pressing relative to theaverage particle diameter before pressing.

Preparation and Evaluation of Aluminum Hydroxide Powder

The aggregated particles obtained above were disintegrated by using ablender and then dried to obtain aluminum hydroxide powder. Filling thealuminum hydroxide powder in an unsaturated polyester resin gave anartificial marble excellent in transparency.

Comparative Example 1

To the first vessel of an apparatus comprising 8 vessels, each equippedwith a stirrer, connected in series was fed continuously asupersaturated aqueous sodium aluminate solution having a temperature of58° C., an Na₂O concentration of 125 g/l, an Al₂O₃ concentration of 121g/l and a molar ratio of 1.7 at a flow rate of 100 parts by weight/hour.In the vessel, part of aqueous sodium aluminate solution was hydrolyzedto obtain (aluminum hydroxide) aggregated particles. The aqueous sodiumaluminate solution containing aggregated particles was transferred tothe second vessel to allow hydrolysis to continue. Succeedingly, theaqueous sodium aluminate solution was hydrolyzed at the third to eightvessel, to obtain aggregated particles. The aqueous sodium aluminatesolution discharged from the eighth vessel had a molar ratio of 3.3. Theaggregated particles obtained were evaluated under the same conditionsas in “evaluation of aggregated particles” described in Example 1. Theresults thus obtained are shown in Tables 1 and 2. TABLE 1 L valueExample 1 63 Comparative Example 1 70

TABLE 2 Average particle diameter (μm) Rate of Before pressing Afterpressing change (%) Example 1 68 30 55.9 Comparative 56 44 21.4 Example1

The aggregated particles obtained above were treated in the same manneras in “preparation and evaluation of aluminum hydroxide powder”described in Example 1. The artificial marble thus obtained did not havea sufficient transparency.

EXAMPLE 2

Preparation of Aluminum Hydroxide Powder

Aluminum hydroxide powder was obtained by repeating the same proceduresas in Example 1 except that there was used a vessel lined as ananticorrosive material with a SUS 304 stainless steel sheet 2 mm inthickness welded to the inner surface of the vessel ranging from thebottom of the vessel to the upper end of the baffle plate, and thatfeeding of supersaturated aqueous sodium aluminate solution wasdiscontinued at the time when the average particle diameter ofaggregated particles in concentrating region 1 reached 40 μm.

Evaluation of Aluminum Hydroxide Powder

The b value of a slurry obtained by mixing 18 g of the powder obtainedabove and 10 g of methyl methacrylate was determined with acolor-difference meter (Z-1001 DP, a trade name, mfd. by Nippon DenshokuKogyo K.K.). The results thus obtained are shown in Table 3.

EXAMPLE 3

Aluminum hydroxide powder was obtained by repeating the same proceduresas in “preparation of aluminum hydroxide powder” described in Example 2except that no stainless steel sheet lining was applied to the vessel.The aluminum hydroxide powder thus obtained was evaluated under the sameconditions as in “evaluation of aluminum hydroxide powder” described inExample 2. The results obtained are shown in Table 3. TABLE 3 Averageparticle diameter (μm) b value Example 2 29 1.5 Example 3 27 3.3

According to the aluminum hydroxide aggregated particles and the processfor producing the same according to this invention, aluminum hydroxideaggregated particles are obtained which are the material for producingaluminum hydroxide powder which in turn can provide, when filled in aresin, a resin composition having a high transparency. The process forproducing aluminum hydroxide powder according to this invention is aprocess which uses the above-mentioned aluminum hydroxide aggregatedparticles, and according to the process, the aluminum hydroxide powdercan be easily obtained. Further, the use of the vessel according to thisinvention makes it possible to produce the aluminum hydroxide aggregatedparticles in a simple and easy manner.

1. A process for producing aluminum hydroxide aggregated particlescomprising the steps of: (a) feeding a supersaturated aqueous sodiumaluminate solution to a vessel, (b) adding aluminum hydroxide seeds tothe supersaturated aqueous sodium aluminate solution to form aseed-added solution in the vessel, (c) stirring the seed-added solutionin the vessel while continuously feeding an additional supersaturatedaqueous sodium aluminate solution into the vessel to hydrolyze thesupersaturated aqueous sodium aluminate solution to obtain aluminumhydroxide aggregated particles and an aqueous sodium aluminate solution,(d) separating the aluminum hydroxide aggregated particles from theaqueous sodium aluminate solution, and (e) continuously discharging theaqueous sodium aluminate solution out of the vessel.
 2. The processaccording to claim 1, wherein the supersaturated aqueous sodiumaluminate solution in steps (a) and (c) has an effective Na₂O (causticNa₂O) concentration of 120-180 g/l, an Al₂O₃ concentration of 120-180g/l and a molar ratio (Na₂O/Al₂O₃) of 1.2-1.8.
 3. The process accordingto claim 1, wherein the aluminum hydroxide seeds added in step (b) havean average particle diameter of 1-2 μm.
 4. The process according toclaim 1, wherein the hydrolysis in step (c) is conducted at 45-80° C. 5.The process according to claim 1, wherein the stirring in step (c) isconducted with a mechanical stirrer having a stirring blade rotating ata peripheral velocity of not less than 0.1 m/s.
 6. The process accordingto claim 1, wherein the separation in step (d) is conducted until thealuminum hydroxide aggregated particles are concentrated to aconcentration of 600 g/l or more.
 7. The process according to claim 1,wherein the aqueous sodium aluminate solution discharged out of thevessel in step (e) has an effective Na₂O (caustic Na₂O) concentration of120-180 g/l, an Al₂O₃ concentration of 60-80 g/l and a molar ratio(Na₂O/Al₂O₃) of 2-3.5
 8. A process for producing aluminum hydroxidepowder, which comprises the step of disintegrating the aluminumhydroxide aggregated particles obtained by the process according toclaim
 1. 9. The process according to claim 8, wherein the disintegrationis conducted with a kneader, blender or extruder.